The traditional implantable cardiac pacemaker includes a pulse generator device to which one or more flexible elongate lead wires are coupled. The device is typically implanted in a subcutaneous pocket, remote from the heart, and each of the one or more lead wires extends therefrom to a corresponding electrode, coupled thereto and positioned at a pacing site, either endocardial or epicardial. Mechanical complications often associated with elongate lead wires, which are well known to those skilled in the art, have motivated the development of cardiac pacing systems that are wholly contained within a relatively compact package for implant in close proximity to the pacing site, for example, within the right ventricle RV of the heart. With reference to FIGS. 1A-B, such a system 100 is illustrated, wherein pace/sense electrodes 111, 112 are formed on an exterior surface of a capsule 101 that hermetically contains a pulse generator 103 (shown in FIG. 1B via a block diagram). FIG. 1A further illustrates tine members 115 mounted to an end of capsule 101, in proximity to electrode 111, in order to secure electrode 111 against the endocardial surface of RV, and electrode 112 offset distally from electrode 111. Capsule 101 is preferably formed from a biocompatible and biostable metal such as titanium overlaid with an insulative layer, for example, medical grade polyurethane or silicone, except where electrode 112 is formed as an exposed portion of capsule 101. An hermetic feedthrough assembly (not shown), such as any known to those skilled in the art, couples electrode 111 to pulse generator 103 contained within capsule 103.
With further reference to FIGS. 1A-B, those skilled in the art will appreciate that system 100, via electrodes 111, 112 and corresponding sense amplifier circuitry 144, has the capability to sense intrinsic ventricular depolarization (i.e. R-waves) and, in the absence of the intrinsic depolarization, to apply stimulation pulses to the RV in order to create paced ventricular depolarization. The amount of energy in each stimulation pulse, or the pulse energy is preferably set at the minimum value necessary to create the paced ventricular depolarization, that is, to capture the heart. A measure of the response of the heart muscle to the stimulation pulse, or the evoked response assures that this pulse energy is sufficient. Those skilled in the art are aware that measuring the evoked response may be confounded by post stimulation pulse polarization, and a number of prior art disclosures are directed toward methods for dealing with this issue. Commonly assigned U.S. Pat. No. 6,144,881 (to Hemming et al.), which is hereby incorporated by reference, in its entirety, describes suitable methods and apparatus (i.e. capture detection circuitry) for reliably and accurately detecting an evoked response in a sensed signal that follows a pacing pulse, by rejecting, or filtering out post pulse polarization contributions to the sensed signal, when the evoked response contribution to the sensed signal meets a minimum amplitude criterion.
Utilizing the apparatus and methods taught by the '881 reference to conduct periodic pacing threshold searches can assure, in most cases, that pacing pulse energies are not greater than necessary to capture the heart. But, particularly in the context of recently developed relatively compact pacing systems, like that shown in FIG. 1A, there is a desire to take additional steps toward even greater system efficiency.